1. Field of the Invention
The present invention relates to a current mirror circuit and a current reference circuit and more particularly, to a current mirror circuit and a current reference circuit that can be operated at an extremely low supply voltage of approximately 1 V.
2. Description of the Prior Art
A current reference circuit producing a constant reference current whose current value decreases in inverse proportion to the ambient absolute temperature is termed an "inversely proportional to absolute temperature (inverse PTAT)" circuit. The constant reference current thus produced has a negative temperature coefficient.
When the supply voltage is comparatively high (for example, 3 V or more), there have been known some inversely PTAT circuits, an example of which is shown in FIG. 1. In the circuit of FIG. 1, a difference current between a bias current for one diode-connected bipolar transistor and another bias current for two diode-connected bipolar transistors is taken out as an output current having a negative temperature coefficient.
Specifically, two npn bipolar transistors Q51 and Q52 and a resistor R51 (resistance: r.sub.51) constitute a first current mirror circuit. The transistor Q51 has a base and a collector coupled together, in other words, it is diode-connected. A current flowing through the resistor R51, i.e., a collector current of the transistor Q51, serves as a reference current. A collector current I.sub.1 of the transistor Q52 serves as a mirror current for the reference current.
The mirror current I.sub.1 is expressed as EQU (V.sub.STB -V.sub.BE51)/r.sub.51,
where V.sub.STB is a supply voltage and V.sub.BE51 is the base-to-emitter voltage (typically, 0.6 to 0.7 V) of the transistor Q51.
Three npn bipolar transistors Q53, Q54 and Q57 and a resistor R52 (resistance: r.sub.52) constitute a second current mirror circuit. The transistors Q54 and Q57 are diode-connected. A current flowing through the resistor R52, i.e., a collector current of the transistors Q54 and Q57, serves as a reference current. A collector current I.sub.2 of the transistor Q53 serves as a mirror current for the reference current.
The mirror current I.sub.2 is expressed as EQU (V.sub.STB -2V.sub.BE54)/r.sub.52,
where V.sub.BE54 is the base-to-emitter voltage (typically, 0.6 to 0.7 V) of the transistor Q54, because the transistor Q57 has the same base-to-emitter voltage V.sub.BE57 as V.sub.BE54.
Two pnp bipolar transistors Q55 and Q56 constitute a third current mirror circuit. The transistor Q55 is diode-connected. The mirror current I.sub.2 of the second current mirror circuit flows through the transistor Q55 as a reference current of the third current mirror circuit. The current I.sub.2 is folded by the third current mirror circuit, thereby producing a constant current -I.sub.2 at a collector of the transistor Q56. Thus, a mirror current (I.sub.1 -I.sub.2) of the third current mirror circuit is produced at the collector of the transistor Q56.
Two pnp bipolar transistors Q58 and Q59 constitute a fourth current mirror circuit. The transistor Q58 is diode-connected. A collector current of the transistor Q58, which is equal to the mirror current (I.sub.1 -I.sub.2), serves as a reference current. A mirror current (I.sub.1 -I.sub.2) serving as a reference current with a negative temperature coefficient is produced at a collector of the transistor Q59.
with the conventional current reference circuit shown in FIG. 1, the bias current for the diode-connected transistor Q51 varies in inverse proportion to the ambient absolute temperature because the base-to-emitter voltage V.sub.BE51 of the transistor Q51 is inversely proportional to the ambient absolute temperature. The temperature coefficient of V.sub.BE51 or the bias current for the transistor Q51 is approximately -2 mV/deg.
Similarly, the bias current for the diode-connected transistors Q54 and Q57 varies in inverse proportion to the ambient absolute temperature because the transistors Q54 and Q57 have the base-to-emitter voltages V.sub.BE54 and V.sub.BE57 that are inversely proportional to the ambient absolute temperature. V.sub.BE54 and V.sub.BE57 have the same temperature coefficient as that of V.sub.BE51, and the transistors Q54 and Q57 are serially connected to each other. Therefore, the temperature coefficient of V.sub.BE54 or the bias current for the transistors Q54 and Q57 is equal to twice as much as that of V.sub.BE51, i.e., approximately -4 mV/deg.
Additionally, the supply voltage V.sub.STB needs to have no temperature coefficient.
The conventional current reference circuit of FIG. 1 has the following problem: Since the transistors Q54 and Q57 are serially connected between the supply voltage V.sub.STB and the ground, the supply voltage V.sub.STB is required to be greater than the sum (approximately 1.2 to 1.4 V) of the base-to-emitter voltages of the transistors Q54 and Q57. As a result, the supply voltage V.sub.STB needs to be equal to or greater than approximately 1.5 V. This means that the conventional circuit of FIG. 1 cannot be operated at a low supply voltage of approximately 1 V.
Moreover, there has been no inverse PTAT circuit operable at a low supply voltage of approximately 1 V.